Disposal of radioactive defense nuclear waste is one of the biggest environmental problems the United States now faces. Over the last several decades over 230,000 m.sup.3 of low level nuclear waste was generated and stored in underground storage tanks located at the Hanford Site in the State of Washington. The low level waste stream consists mostly of sodium nitrate and nitrate salts in alkaline liquid/slurry form.
Reprocessing of either spend nuclear fuel or weapons material results in liquid waste which must be reduced in volume and consolidated to permit safe disposal. The current practice is to dehydrate the liquid waste by heating, then to consolidate the residue by either calcination or vitrification at high temperatures. In the past, defense waste was neutralized in order to precipitate metallic hydroxides. This product can be converted into a vitreous waste form using conventional glass forming technology.
The ultimate suitability of vitreous waste forms is suggested by the durability of rhyolytic obsidian and tektite natural glasses during millions of years in a variety of geologic environments. Unfortunately, these chemically durable, high-silica glasses pose problems as a practical solid-waste form, when made using conventional continuous vitrification processes. Because of the required high fluxing temperatures (.about.1350.degree. C.), additional off-gas scrubbing capacity or other absorbent procedures are needed to deal with the volatilization losses of radionuclides such as iodine, cesium, and ruthenium. High fluxing temperatures also shorten furnace life and can create problems with the materials into which the molten glass is cast, such as the sensitization of stainless steel to stress corrosion cracking. As a consequence of these limitations, most nuclear waste glass formulations have substantially lower silica content than either natural obsidians, nepheline syenite, or commercial "Pyrex" glasses. Less silica or alumina and more fluxing agent (e.g., Na.sub.2 O, K.sub.2 O or B.sub.2 O.sub.3) lowers the glass working temperature (to 1000.degree.-1200.degree. C. for most waste glasses) and raises the waste loading capacity. However, this also results in lower chemical durability in most aqueous environments and, particularly for borosilicate compositions, in less resistance to devitrification.
U.S. Pat. No. 4,377,507 to Pope, et al. ("Pope '507"), U.S. Pat. No. 4,376,070 to Pope, et al. ("Pope '070"), and U.S. Pat. No. 4,759,879 to Cadoff, et al. ("Cadoff") disclose immobilizing nuclear waste in glass produced from, amongst other things, a glass-forming silicon compound of the formula SiR.sub.m (OR').sub.n X.sub.p or Si(OSiR).sub.4 and a glass-forming aluminum compound with the formula AlR'.sub.a (OR').sub.r X.sub.s or Mg(Al(OR).sub.4).sub.2 or Al(OH).sub.3 which are each hydrolyzed in alcohol and water, then mixed together. Nuclear waste can then be added as a solid or as an aqueous solution. The mixture is heated to form a gel and dried. The resulting vitreous granules may be sintered at 800.degree.-900.degree. C. (see Pope '507) or melted to form ingots (see Pope'070 and Cadoff). These techniques are not highly satisfactory, because the use of high temperature melting or sintering results in the volatilization of radioactive materials. When melting, high temperature equipment, which is limited in its ability to handle large quantities of nuclear waste, must be utilized. Further, the processes of Pope '507, Pope '070, and Cadoff are not fully able to solubilize nuclear waste materials and, as a result, do not form a homogeneous glass able to maintain nuclear waste materials in an immobilized state.
The present invention is directed to overcoming these deficiencies.